The present invention relates to a method for producing FRP screw-like fastening elements. The screw-like fastening elements include various screws and rivets. These screws and rivets may be preferably used for aircraft or the like.
Screws made of plastic material have been utilized in various field, for example the aircraft field, because of their lightness and high corrosion resistance. As the name FRP (fiber reinforced plastic) suggests the plastic material is frequency reinforced with fibers in order to improve the mechanical strength. Carbon fibers are mainly utilized as the reinforcement fibers.
Conventionally, FRP screws are manufactured in such a method that a rod plastic material including carbon fibers embedded along the length of the rod is prepared, and then a thread or threads are formed on an outer peripheral surface of the material by machining.
However, when the thread or threads are formed, the fibers which are embedded at a radial outer portion to be the thread groove are torn to pieces. Thus the thread portion has a low mechanical strength in comparison with that of the bulk of the screw. The thread portion does not have enough reinforcement advantage, so it is fragile and can be broken sometimes.
In this regard, another manufacturing method for FRP screws was proposed. As shown in FIG. 1, first, high strength fibers 2 are applied into a matrix of thermoplastic resin 4. The thermoplastic resin 4 is formed by extrusion molding or drawing into a rod material 6. In this time, fibers 2 are arranged in a row along a direction of the length of the rod material 6. The thermoplastic resin 4 is a light and strong material, e.g., a polyether-etherketone resin. The fibers 2 are, e.g., carbon fibers. The material 6 preferably includes carbon fibers 4 constituting 30-80% of the weight, and more preferably 60-70%.
Next, the rod material 6 is cut to have a prescribed length and disposed into a metallic mold 8 as shown in FIGS. 2 and 3. The mold 8 comprises a pair of half mold members 10 and 12. Each of the mold members 10 and 12 includes a semi-circular mold surface which has small grooves carved therein, the grooves forming a thread and threads when the half separated mold members 10 and 12 are combined together. As shown in FIGS. 4 and 5, the material 6 is heated and pressed between the half mold members 10 and 12, to form a screw 14 which has a thread and threads shaped by the small groove. Then, mold members 10 and 12 are separated again to take out the manufactured screw 14. The thread portion includes fibers 2 which are not damaged, having sufficient strength.
However, in the above method, if the plastic material 4 is excessive, the manufactured screw 14 will have burrs 16, so that it is necessary to deburr or reject the screw. If the plastic material is insufficient, the screw will be defective and must be rejected.
Furthermore, the pressure to form the screw 14 is limited by the capacity of the mold and the volume of the material. The pressure is also limited in order prevent the occurrence of deburr 16. Therefore, the material 6 is not subjected to a large pressure. If the material 6 includes defects such as cavities, the cavities may remain in the manufactured screw 14. Consequently, the manufactured screw 14 sometimes does not have a prescribed strength.
In addition, the above-described method is not suitable for producing a screw with a head, because the rod material 6 is originally of a uniform cross section.